1. Field of the Invention
The present invention relates to a process for depositing diamond by chemical vapor deposition, more particularly to a process for depositing diamond without using conventionally-used hydrogen, which is an explosive gas, as a reaction gas, and to a process for depositing diamond at temperature below 400.degree. C.
2. Description of the Prior Art
Diamond, which is the hardest substance (about Hv 10,000 kg/mm.sup.2) in the world, has very high thermal conductivity (about 24 watt/.degree.C.-cm), and good insulating properties (about 10.sup.13 -10.sup.16 .OMEGA.cm/15.degree. C.). In addition, it is transparent within the ultra-violet, visible and infra-red spectrum. Diamond is also resistant to chemical corrosion, low in friction, and high in sonic propagation speed. Therefore, diamond is a valuable material with many special characterstics not found in other materials. For example, diamond may become semi-conductive if a small amount of phosphorols or boron is added to a diamond film. As a result, diamond is regarded as an important material in electronic and defense industry application.
Synthetic diamond was previously used in the manufacture of tools. In the past few years, techniques for producing artificial diamond have improved greatly, expanding the range of applicability of synthetic diamond into such industries as optical instruments, semi-conductor elements, VLSI integrated circuit heat sinks, horn-like vibrating plates, the reflection resistant film of solar batteries, and various other mechanical tools.
Diamond films are usually synthesized from gas mixtures that include a small amount of hydrocarbon in hydrogen which is an explosive gas, and the typical growth temperature is 700.degree.-1000.degree. C. For many applications, it is important to grow diamond films at low temperatures (below 500.degree. C.), because many substrate materials, such as GaAs, plastics, ZnS and MgF.sub.2, are unstable at high temperature.
Deposition of diamond films at low temperature can be achieved by altering deposition techniques and reaction gases.
For example, Liou et al. use microwave plasma chemical vapor deposition (MPCVD) to deposit diamond. The reaction gases are H.sub.2 /CH.sub.4 /O.sub.2. Diamond can be deposited at temperatures above 400.degree. C. (Appl. Phys. Lett. 56(5), p. 437(1990)).
Yasuchi et al. also synthesize diamond by MPCVD. The reaction gases are CO/O.sub.2 /H.sub.2. It is confirmed that diamond can be grown at temperatures above 400.degree. C. (Surface and Coatings Technology, 47(1991), p. 1).
Wei et al. disclose that diamond films can be grown by magneto-microwave plasma CVD with CH.sub.4 +CO.sub.2 /H.sub.2 gas mixture at temperature above 500.degree. C. (Journal of Crystal Growth, 99(1990), p. 1201).
All the above processes for depositing diamond have the common problem that a large amount of H.sub.2 which is an explosive gas is required, thus causing possible danger. Another problem is that the deposition must be carried out at temperatures higher than 400.degree. C., thus requiring high energy and limiting the substrates which can be used.